This invention relates to the fabrication of semiconductor devices and, in particular, to the formation of metal layers on semiconductor devices.
In manufacturing integrated circuits, one must provide conductive paths interconnecting various devices in the chip. These conductive paths may comprise a variety of metal or metal alloys such as tungsten silicide, polysilicon, or aluminum. In a typical process, a layer of aluminum is deposited and a layer of photoresist is deposited over the aluminum. The photoresist is patterned and then the aluminum is etched in a like pattern.
A problem occurs when it is time to form the next layer or layers over the conductive paths. The aluminum layer can be one micron (1000 nm.) thick, a considerable thickness relative to other layers. Further, the thickness is on the order of the smallest device geometry. Thus, one has what is known as a "step coverage" problem, e.g. forming a second conductive path at a right angle to the first and maintaining continuity over the step.
The problem of step coverage can also arise simply from the subsequent layer having a discontinuity at the outside corner of the step. To avoid this problem, various means have been employed to minimize the effect of the step. One way is simply to make the next layer so thick that discontinuities will not arise. This solution is frequently or even usually not available since layer thickness is determined by other parameters. An alternative is to doubly etch the aluminum layer, i.e. with and without photoresist. The problem with this alternative is that it adds several steps to the process flow and thins the conductive layer. Another alternative is to use an isotropic etch. In this case, uniformity becomes a problem since the undercutting of the photoresist tends to be erratic.
It is known in the prior art to etch aluminum in a plasma using carbon tetrachloride and chlorine, as shown in U.S. Pat. Nos. 4,182,646 and 4,341,593. It is also known, but not recommended, to etch aluminum in a plasma with silicon tetrachloride and nitrogen, as shown in U.S. Pat. No. 4,373,990. It is also known to erode a mask to obtain a tapered hole in the etched layer; e.g. for tapered holes in oxide see "Tegal Plasma Seminar Proceedings", (1983), pp 5-11 and 17-24.
In view of the foregoing, it is therefore an object of the present invention to provide an improved process for etching aluminum in a plasma.
Another object of the present invention is to provide a process for forming metal layers with controllable edge taper.
A further object of the present invention is to provide an improved plasma etch process using silicon tetrachloride.